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MT834
Unit 1
The Web and
the Internet
080
Course team
Developer:
Jenny Lim, Consultant
Designer:
Chris Baker, OUHK
Coordinator:
Dr Li Tak Sing, OUHK
Member:
Dr Andrew Lui Kwok Fai, OUHK
External Course Assessor
Prof. Mingshu Li, Institute of Software, Chinese Academy of Sciences
Production
ETPU Publishing Team
Copyright © The Open University of Hong Kong, 2004.
Reprinted 2008.
All rights reserved.
No part of this material may be reproduced in any form
by any means without permission in writing from the
President, The Open University of Hong Kong.
The Open University of Hong Kong
30 Good Shepherd Street
Ho Man Tin, Kowloon
Hong Kong
Contents
Overview
1
Objectives
2
Introduction
3
What is the Web?
4
Design and structure
HTML
URLs
HTTP
The Internet
Design and structure
Communication protocols
Internet Protocol (IP)
Ports
Transmission Control Protocol (TCP)
Web servers
Role of browsers and servers
Installing a local Web server
5
6
9
15
18
19
21
22
28
29
32
32
33
Summary
38
Suggested answers to self-tests
39
References
42
Unit 1
Overview
Welcome to Unit 1 of MT834 Web Server Technology.
Please be reminded that you should have read the MT834 Course Guide
by now. It’s also a good idea to browse the course website through the
Open Learning Environment (OLE). The course website offers
interesting information and activities associated with each unit. You will
find that this course includes theoretical concepts as well as hands-on
experimentation. As a graduate student, you are also encouraged to visit
the University’s award winning Electronic Library. If you have done all
these things, you are ready to get started with this first unit.
I hope you are as excited as I am by the new era of the information age.
Without a doubt, the Web — together with the Internet — plays a most
important role in allowing information to be accessed easily and
instantly.
To understand how the Web works, you first need to have a good
understanding of the underlying network which serves as its delivery
medium — the Internet. Here are the aspects of the Internet we shall
examine in this unit:
•
the physical network;
•
design and structure of the Internet;
•
Domain Name Service (DNS); and
•
Transmission (or Transport) Control Protocol/Internet Protocol
(TCP/IP).
Next, we’ll discuss the technologies used to provide the World Wide
Web service over the Internet:
•
HyperText Transfer Protocol (HTTP);
•
Uniform Resource Locator (URL);
•
Web client and server software; and
•
HyperText Markup Language (HTML) documents.
To learn these concepts, you will read selected online readings and
conduct a series of hands-on exercises on your local computer. You will
install a Web browser, create your own HTML document, and then
install and configure an Apache Web server to serve the HTML
document.
This first unit of MT834 Web Server Technology is expected to take you
four weeks (or about 30 hours of study time) to complete. Please plan
your time carefully. As you work through the unit, you will need to refer
to online readings and activities on the MT834 Web Server Technology
course website.
You may begin Unit 1 now by reading the unit’s learning objectives.
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Objectives
By the end of this unit, you should be able to:
1
Explain how the World Wide Web, Domain Name Service, FTP and
other applications are made available over the medium of the
Internet.
2
Discuss the design and structure of the Internet, and the use of
TCP/IP as its transmission protocol.
3
Create basic HTML documents and serve these documents from a
local Web server.
4
Describe the role of Web browsers and Web servers.
Unit 1
Introduction
As a distance education student, the World Wide Web is an integral part
of your life. In courses such as this one, you rely on it to communicate
with tutors and fellow students, download study materials, and view
online resources. This experience is bound to make you very familiar
with the Web from an end-user’s point of view, that is, from the
perspective of someone who requests information via a Web browser.
But what really goes on behind the scenes after you click on a hyperlink?
How does your Web browser locate the document that you want and how
does this document get transmitted to your machine? Where are these
documents stored and how are they organized? For someone who is
about to embark on a course in Web server technologies, you need a
deeper understanding of the Web and the Internet beyond that of an
experienced Web user.
In this unit, we will take our first steps towards gaining this knowledge.
We will start with an in-depth discussion of the design principles,
technologies and protocols behind the Web and the Internet. These
fundamental concepts will underlie the Web server technologies to be
discussed throughout the course.
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What is the Web?
Computers have been connected to the Internet since the 1970s, and data
exchange between networked computers has been around for just as long.
However, the launch of the World Wide Web in the early 1990s offered
the prospect of something totally new. It allowed the entire Internet to be
viewed as a single information space, where users accessing data could
move seamlessly and transparently from machine to machine by
following links. Before the Web, individual Internet computers had
windowing systems and graphical capabilities, but networking
applications such as email and FTP were still text-based. The ease,
convenience and graphical nature of the Web has made it the ‘killer’
application of the Internet.
The Web, as it is commonly called, is a collection of interlinked
information that is accessible through a worldwide network. It is a digital
‘information space’ with a means for users to access and retrieve
documents from it. Here are the components of the Web (also shown
figure 1.1 below) which make all this information storage, organization
and retrieval possible:
1
Information in the form of multimedia documents. Multimedia means
that these documents can be composed of text, images, animation,
audio, video, and other types of content.
2
Computers where these documents are stored (known as servers or
providers) and computers from which these documents are accessed
(known as customers or clients).
3
A networking medium which connects these clients and servers so
that data can travel between them, namely, the Internet. The Web
was built to run ‘on top’ of the Internet, which exists independently
of the Web.
4
Web client and server software which allows webpages on any Web
server to be accessed from around the world. From the user’s point of
view, all that’s needed is a Web browser and an Internet connection
in order to get on the Web. From the information provider’s point of
view, he/she needs a Web server which is connected to the Internet.
Image of
Web
browser
Figure 1.1
Internet
Overview of the Web and its components
Image of
Web server
with
documents
Unit 1
For the rest of Unit 1, we will discuss the technologies, protocols and
standards that are used by these different Web components.
Design and structure
Tim Berners-Lee proposed the World Wide Web in 1989 because he
wanted a better way of sharing and retrieving information among the
people who worked at the CERN (European Laboratory for Nuclear
Research) office in Geneva, Switzerland. When he was designing the
Web, these were some of his goals:
•
To allow access to different kinds of information stored on disparate
computing platforms. Common protocols had to be used to provide a
bridge between different computer operating systems and networks.
•
To use hypertext, or nonlinear text, that allows related documents to
be tied together via ‘active links’, and that users can ‘follow’ by
clicking on the links. The Web browser then fetches and displays the
document pointed to by the link.
•
To decentralize control and access. In order to get on the Web, all
that was needed was access to the networking medium (e.g., the
Internet) and software to retrieve and view the documents on it (e.g.,
the browser). There was no central node or computer to which
everything had to be connected.
The Web’s architecture follows a standard client-server model. In this
model, a user relies on a program (the client) to connect to a remote
machine (the server), where the requested resource is stored. Possible
resources could be text files, multimedia documents or dynamically
generated pages.
Web clients, such as Internet Explorer and Firefox, know how to present
data but do not need to know the details of how this data is stored or
generated. Web servers, such as Apache and Internet Information Server
(IIS), know how to extract data, but are ignorant of the details of how it
will be presented to the user.
Tim Berners-Lee came up with three important new technologies for
creating the Web:
1
HyperText Markup Language (HTML);
2
Uniform Resource Locators (URLs); and
3
HyperText Transfer Protocol (HTTP).
These were based on ideas which emerged in the last few decades.
However, the technology needed to make hypertext systems a reality was
only brought together in the early 1990s, with the birth of the Web. In the
remaining part of this section, we will discuss these three technologies in
more detail. Now answer the following questions to test your
understanding of the Web’s design and structure.
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Self-test 1.1
1
The Web uses the client-server architecture model. What does this
mean and how can clients and servers become part of the Web?
2
What is the relationship between the Web and the Internet?
HTML
HTML, or HyperText Markup Language, is the markup language used to
create documents on the Web. A markup language allows authors to
highlight portions of the content and assign meaning to these portions by
tagging them.
HTML documents are plaintext or ASCII files that can be created using
any text editor on any machine. Tim Berners-Lee chose the plaintext
format because it could be understood by all computers, regardless of
their operating system or hardware platform. This is an important factor
behind the universality and accessibility of the Web.
HTML documents contain a combination of text and markup tags. The
markup tags specify the logical structure and organization of a document,
for example, which parts belong to the head and body of the page. Here
are the basic tags which must be found in every HTML document.
<HTML>
<HEAD>
<TITLE> page title </TITLE>
</HEAD>
<BODY> body of the document </BODY>
</HTML>
Figure 1.2
Basic HTML document tags
Aside from these basic tags, there are other tags which can be used to
mark up parts of the Web document body, such as the paragraphs,
headings, lists, quotes, definitions, citations, etc.
The interpretation of these marked elements are left to the browser. This
choice was made because the same HTML document may be viewed by
different browsers of varying abilities.
Here is a more detailed example of the elements that may be found in an
HTML document.
Unit 1
HTML source
<html>
<head>
<TITLE>Learning HTML</TITLE>
</head>
<body>
<H1>HTML is Easy To Learn</H1>
<P>Welcome to the world of HTML. This is
the first paragraph. <b>This text is bold.</b>
</P>
<P>And this is the second paragraph. This
text is <em>emphasized</em>.</P>
<P>Here are 3 reasons to learn HTML:</P>
<ul>
<li>You can build your own home page.
<li>You can start an online business.
<li>You can share your photo albums.
</ul>
</body>
</html>
How it is displayed by my browser (Internet Explorer 5.5)
Figure 1.3
Elements that may be found in an HTML document
Aside from defining the structure of the document, markup tags can also
be used to identify the hyperlinks within the page. Just as there are
HTML tags for representing formatting directives, there are HTML tags
called anchor tags for representing HTML links, i.e., anchors, to other
Web resources. When you use your mouse to point-and-click an HTML
document and a new document or other multimedia resource pops up,
you are using HTML anchors and HTML’s hypertext capabilities.
The text The Open University of Hong Kong with a hypertext link to the
OUHK’s homepage would be written like this:
<A HREF="http://www.ouhk.edu.hk"> The Open
University of Hong Kong</A>
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When displayed by your Web browser, the anchor is highlighted and
underlined. It would look like this:
The Open University of Hong Kong
We will discuss how a Web address, or Uniform Resource Locator
(URL) is formed a little later in the section titled ‘URLs’.
Although you’ve now had a basic introduction to HTML, we’ve only
covered a very small portion of its available tags. From its origins as a
structural markup language, HTML has also grown to include formatting
tags which describe how elements should appear (e.g., in what colours
and sizes).
If this is your first exposure to HTML, the following reading will give
you a good foundation in the important tags that are needed to build
webpages. The first item in the reading is a highly readable introduction
to HTML. I’ve also provided a shorter item, item 2, that you can skim
through if you want to refer to some basic guidelines for constructing a
simple HTML document.
Reading 1.1
1
‘A Beginner’s Guide to HTML’, section Markup Tags
http://www.put.com/HTMLPrimer.html#MT
2
‘HTML for the conceptually challenged’,
http://www.arachnoid.com/lutusp/html_tutor.html.
When you have completed Reading 1.1 you should be able to construct
an HTML document containing HTML tags for these elements:
1
bold text;
2
italicized text;
3
underlined text;
4
paragraph;
5
heading;
6
hypertext link or hyperlink;
7
inline image;
8
background graphic;
9
font colour; and
10 background colour.
Unit 1
Activity 1.1
The files needed for the following activity can be found on the OLE.
ABC Books has just decided to establish an online presence and they
have given you a document containing the information they wish to
appear on their homepage.
1
Please download the document called abc_home.zip which
contains abc_home.doc and three graphic files.
2
Convert this information in the word file into an HTML document
called abc_home.html. Code this page by hand, using the tags
you’ve learned in this section. The three graphic files are to be used
in the HTML document.
Note: If you have difficulty with this or any of the other activities, please
seek help on the MT834 discussion board, or contact your tutor.
URLs
In the previous section, you learned that HTML uses the anchor tag <A>
in order to tell the Web browser where an information resource is located
on the Internet. Uniform Resource Locators (URLs) are used within
anchor tags to specify a unique online address for each resource, just as
street addresses express the unique location of a place in our physical
world.
From your travels on the Web you are probably familiar with the basic
form of a URL, as follows:
http://hostname/path/filename.html
A Web URL includes the protocol name (http), followed by a colon and a
double slash (://), followed by the Web server name or address, and the
location of the file on the server. If a filename is not specified, then the
server will return the default homepage. Using the above example, you
should recognize that the URL for the Open University of Hong Kong
homepage would be http://www.ouhk.edu.hk.
The next reading gives you more details on how different kinds of URLs
can be coded in HTML pages.
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Reading 1.2
A Beginner’s Guide to HTML, section Linking
http://www.put.com/HTMLPrimer.html#LI2.
Now that you’ve got an idea of how webpages and hyperlinks are coded
using HTML, let’s take a closer look at the software that is used to
interpret and display HTML documents, namely, the Web browser.
Activity 1.2
This activity requires you to have two Web browsers installed on your
machine so that you can contrast how each Web browser handles
common browser functions. The browsers used for the screen shots in
this activity are Internet Explorer 6 and Firefox.
You may use any reasonably current version of these two browsers. If
you need to upgrade to a later version or you need to acquire new
browsers, you can download
•
Firefox at
http://www.mozilla.com/firefox/all
•
Microsoft Internet Explorer Web browser at
http://www.microsoft.com/windows/ie/downloads/default.mspx.
Follow the instructions for downloading and installing a new version of
each Web browser for your operating system.
Aside from displaying HTML documents, Web browsers also come with
common menu-based functions. Functions are accessed via menu bars
along the top of the browser window. An example of a common function
would be configuring the browser to use certain fonts, text sizes or
languages. Browsers from different vendors may label these menu
options differently or place them in slightly different locations. Let’s
compare and contrast how Firefox and Internet Explorer handle some
common functions.
1
Viewing or altering the configuration settings of the Web browser
•
For Internet Explorer: Tools Æ Internet Options.
•
For Firefox: Tools Æ Options.
Unit 1
Figure 1.4
Internet Options in Internet Explorer
You will see common configuration settings for the browser such as font
colours and language preferences. We will be altering some of these
configuration settings in future units. Let’s change the default font colours.
Figure 1.5
2
Changing default colours in Internet Explorer
Viewing the properties of an image
Point each Web browser at the URL for the Open University of Hong
Kong’s homepage at http://www.ouhk.edu.hk. Position your mouse
over an image in the webpage, then right-click on the mouse
select Properties.
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Both browsers display the URL, the size and the dimension of the
image. IE also shows the creation date of the image.
Here’s what the Properties window looks like after I right-click on
an image from OUHK’s homepage.
Figure 1.6
3
The image Properties window
Viewing the source of the HTML document
Point each Web browser at the URL for the Open University of Hong
Kong’s homepage at http://www.ouhk.edu.hk.
•
For Internet Explorer: View
•
For Firefox: View
Source.
Page Source.
You should be able to recognize some HTML tags as well as HTML
hypertext anchor tags. You will also see a variety of HTML tags
indicating some interactive technologies such as JavaScript or Java.
Ignore these complex HTML tags and constructs for now; we will
cover these concepts in later units.
Can you find the basic tags <HTML>, <HEAD>, <TITLE>, and
<BODY> in the HTML source of the Open University of Hong
Kong’s homepage?
Unit 1
MIME types
The HTML documents we’ve seen so far are all written and encoded
using the ASCII or plaintext format. However, URLs not only point to
HTML documents, but may also point to multimedia resources that are
encoded in formats other than text. For example, images are usually in GIF,
JPEG, or PNG format, while audio files might use MPEG, AU, or MP3.
It is impossible for Web browsers to have the built-in capability to render
every media format that is available. So how does the browser know how
to handle those formats for which it does not have a native rendering
capability?
Multipurpose Internet Mail Extensions (MIME) types are the answer.
MIME is an international standard that defines the rules for exchanging
information that uses non-ASCII text encoding. It enables the client to
know what type of file to expect and what software to use to interpret the
file, in case the client is not capable of understanding this type of encoding.
MIME also defines a standard set of names for the different data formats
that could be transmitted over networks. The names come in two parts:
the file type, followed by a slash (/), followed by a subtype. The
following table lists some of the better-known MIME file types and
subtypes, together with helper applications that can be invoked by the
client to display them.
Table 1.1
Better-known MIME file types and subtypes
MIME file types
and subtypes
File
extensions
Description
Application/msword
doc
Microsoft Word document
Application/zip
zip
Compressed file that can be
opened using PKZip, WinZip or
other file compression software
Image/jpeg
jpeg,
jpg
JPEG image file, which can be
opened natively by the browser
and by graphics editors such as
Adobe Photoshop, Macromedia
Fireworks, etc.
Image/gif
gif
GIF image file, which can be
opened natively by the browser
and by graphics editors such as
Adobe Photoshop, Macromedia
Fireworks, etc.
Video/quicktime
mov, qt
Quicktime movie file
Audio/midi
midi
LiveAudio
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The Web server can be configured to send the correct MIME type to the
Web browser along with the requested resource. The Web browser
examines the MIME type and displays the resource if it has the native
capability to do so. If not, it can be configured to launch the appropriate
helper application that can handle this resource. This is illustrated in the
next activity.
Activity 1.3
Let’s compare the approach used by Firefox and Internet Explorer in
setting up and recognizing MIME types. This activity also illustrates how
different vendors implement the same features in their software in
different ways, further reinforcing what you’ve seen in the previous
activity.
1
Using both browsers, visit the OUHK website and click on the
Students link. Then, choosing the Information tab, go to
Prospectus and then select View online. This link will take you to
the Adobe Acrobat document (PDF) that contains the University
prospectus. How did both browsers handle this file type?
2
Now let’s see how Firefox and IE were configured to use Adobe
Acrobat to render the PDF file.
For Firefox, the configuration screen can be accessed via these menu
options: Tools Options Downloads Plug-Ins.
Figure 1.7
Viewing MIME types in Firefox
Unit 1
For Internet Explorer, the MIME types are embedded in the Windows
operating system. On XP, the MIME types can be found by clicking on:
My Computer Tools Folder Options File Types. The resulting
display shows a list of all the registered MIME types on the system.
Figure 1.8
Viewing MIME types in Internet Explorer
HTTP
One of the most important concepts when learning how clients and
servers communicate over a network is protocol.
Douglas Comer defines a protocol as ‘a formal description of message
formats and the rules two or more machines must follow to exchange
those messages’ (cited in Connected: An Internet Encyclopedia;
Programmed Instruction Course, ‘Protocols’, http://freesoft.org/CIE/
Course/Section1/3.htm). Protocols usually exist in two forms. First, they
exist in a textual form for humans to understand. Second, they exist as
programming code for computers to execute.
Whenever a computer needs to send data to or receive data from another
host, a protocol is needed to specify how every bit of every message
should be written and interpreted. The protocol also describes how to
handle error conditions. HyperText Transfer Protocol, or HTTP, is the
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protocol used by the World Wide Web service. HTTP describes how
Web clients make requests for information and how servers respond to
these requests. Other services, such as email and FTP, also have their
own protocol specifications.
HTTP is an openly published standard protocol, which allows browsers
and servers written by different vendors to communicate with each other,
as long as their software speaks and understands HTTP.
Like most network protocols, HTTP uses the client-server model: an
HTTP client opens a connection and sends a request message to an HTTP
server; the server then returns a response message, usually containing the
resource that was requested. After delivering the response, the server
closes the connection. This makes HTTP a stateless protocol which does
not retain any connection information between transactions.
HTTP messages are text-based and can be made up of several lines.
Figures 1.9 and 1.10 are sneak previews of what HTTP messages can
look like. Figure 1.9 shows an HTTP request sent by a Web client. The
first line describes what the client wants to do (e.g., get a document) and
includes the URL of the desired document. This could be followed by an
optional number of headers. In figure 1.10, you see that successful
requests return a status code of 200 in the first line. There could be other
response headers attached, followed by the requested document itself.
GET /index.html HTTP/1.0
Host: www.ouhk.edu.hk
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0;
Windows NT 5.0)
Figure 1.9
Sample HTTP request sent by a Web client
HTTP/1.1·200·OK
Date:·Fri,·10·Oct·2003·07:44:55·GMT
Server:·Apache/1.3.22·(Unix)·mod_jk/1.1.0·mod_ssl/2.8.5·OpenSSL/0.9.6b
Last-Modified:·Wed,·18·Jun·2003·09:43:11·GMT
ETag:·"250b81-5a62-3ef0342f"
Accept-Ranges:·bytes
Content-Length:·23138
Connection:·close
Content-Type:·text/html
<html>
<head><title>Sample Response</title></head>
<body>Hello, world !</body>
</html>
Figure 1.10 Sample HTTP response sent by Web server
The rules for constructing valid HTTP messages will be explained in
more detail in Unit 2. However, the next activity allows you to
Unit 1
experiment with different requests and examine the HTTP messages that
are generated.
Activity 1.4
In this activity, you view the HTTP messages exchanged by your Web
browser and the Web server using the online HTTP viewer at
http://www.rexswain.com/httpview.html.
1
Here are some pages to access: www.ouhk.edu.hk and
www.yahoo.com.
2
For each HTTP request, identify where the URL is located.
3
For each HTTP response, identify where the status code and the
HTML document is located.
You should now understand the basics of HTML, URLs, hypertext, and
the multimedia nature of Web documents. Test your knowledge by
completing the following self-test. Suggested answers are provided at the
back of the unit.
Self-test 1.2
1
What three new technologies were created to build the Web? What
does each of these technologies do?
2
What is hypertext? What are some of the characteristics of hypertext?
3
What is a URL? Give an example of a URL.
4
How do you represent a hypertext link in HTML?
5
How do you represent an inline image in HTML?
6
What HTML tags should always be included in an HTML document?
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The Internet
The Internet is a network. More accurately, the Internet is a network
made up of many connected and cooperating computer networks. All
these networks communicate using the same methods or protocols. These
interconnected networks spanning the entire globe are called an
internetwork, hence the term Internet.
You can visualize the Internet as a giant, global plumbing system, similar
to the pipes that bring water to your homes, except it’s used to carry
digitized data. The Internet in itself is useless if there is no traffic
travelling on it, and as you’ve seen previously, email and the Web are the
most popular services which use the Internet as their transport medium.
The Internet was a product of the Cold War. It began as an effort to
create a communication network that could withstand nuclear attacks.
The idea behind the new network was that even if a section of it was
destroyed, messages could still be delivered by redirecting them over
sections that were still intact. None of the existing networks of the time
could handle this requirement, including the telephone system which was
vulnerable to attacks on its switching stations. Before the Internet,
computers had to be directly connected to each other if they needed to
communicate. Messages sent from host computer A to host computer B
could only travel on a single, fixed route. There were no alternative paths
that could be used in case this direct route was destroyed.
Therefore, a new type of network was designed and built that could
fulfill the requirements of a wartime network: one that we now call the
Internet.
The following online reading outlines the history of the Internet and its
unique design which allows it to handle failure and re-route traffic
around trouble spots.
Reading 1.3
Ruthfield, S (1995) ‘The Internet’s history and development: from
wartime tool to the fish-cam’, ACM Crossroads,
http://www.acm.org/crossroads/xrds2-1/inet-history.html.
Note: You do not have to remember the historical facts and
organizations in the article such as RAND, MIT, UCLA,
ARAPANET and Pentagon.
There are two important characteristics of the Internet which stand out in
Reading 1.3.
First, the Internet is decentralized. There are no top-level computers or
routes that can fail and stop the operation of the entire network. There
Unit 1
can also be multiple, redundant routes between two computers, so if one
route becomes unavailable, other routes can still be used.
Second, the Internet is a packet-switching network. Messages are broken
into discrete units called ‘packets’ which contain the addresses of the
source and destination host. These packets are routed separately through
intermediate nodes, hopping from one to another until they reach their
intended destination. If a packet fails in traversing the network via a
particular node, it is simply resent, again, and again, across another
network path or ‘route’ until it reaches its destination. When all the
packets have arrived at their destination, they are reassembled to form the
original message. This design makes it possible for two computers to
communicate with each other even if there is no direct, dedicated
network cable running between them.
B
WWW
client
G
C
A
D
F
H
WWW
server
E
Figure 1.11 Packets travel from the source to the destination via intermediate
gateways, or routers, in a packet-switching network such as the
Internet
Nobody owns or controls the Internet, although companies and even
governments might be responsible for building and maintaining portions
of it. Rather, any network that voluntarily implements the Internet’s
standard protocols may participate in it. Many Internet providers not only
adhere to these standards, but also open up their networks to data traffic
from the general public. The voluntary interconnections and cooperation
between these network providers make the global Internet possible.
Design and structure
The Internet is a network of networks. At the very lowest level these
networks consist of a set of physical network hardware and low-level
communication software. This physical network layer is the lowest layer
where a network connection takes place. Two physical nodes or
computers on the network ‘connect’ to exchange messages in some form.
A network connection can have a variety of physical forms as shown in
figure 1.12.
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Satellite and
antenna
Ethernet local area network
Ethernet
PC
Cable modem
Ethernet
56K modem
and telephone line
Microwave and
antenna
modem
Cable
telephone
line
Set-top box
Interactive Set-top box
Cable
The Internet
The Internet
Figure 1.12 Physical connections to the Internet
Data is transmitted over various carriers, such as telephone lines, cable
TV wires, and satellite channels. When you dial in to your Internet
Service Provider (ISP), your computer actually becomes a node on the
ISP’s network, and from here, you gain access to the Internet.
Each network has its own low-level communication software (such as
Ethernet, FDDI, X.25, IBM token ring, or ATM) so that the specialized
network hardware components can ‘talk’ to one another. The Internet
network software operates on top of these communication layers. The
magic of the Internet is how these very different computer networks
cooperate to form one internetwork — the Internet.
The next reading describes the networks and the interconnections
between them which form the Internet’s infrastructure.
Reading 1.4
Howstuffworks, ‘How Internet infrastructure works’ (by Jeff Tyson),
http://computer.howstuffworks.com/internet-infrastructure.htm.
Note: Please only read the first three sections: (1) ‘Introduction to
how Internet infrastructure works’, (2) ‘A hierarchy of networks’,
and (3) ‘Bridging the divide’.
Now complete the following self-test to assess your understanding of the
Internet’s design and structure.
Self-test 1.3
1
Describe two design characteristics of the Internet that make it wellsuited for carrying wartime communications.
2
What is the relationship between the networks that make up the Internet?
Unit 1
Communication protocols
The Internet has the ability to provide a bridge between different
computer operating systems and networks. This is why Tim Berners-Lee
was interested in providing the World Wide Web service over the
Internet.
It is relatively easy to add a new network to the Internet. The
communication protocols used are built on standards that are open and
publicly available. Owners of diverse physical network types or different
computer operating systems can join the Internet simply by implementing
or purchasing the appropriate protocols.
The task of transmitting data across the Internet is divided up among
several network protocols. Protocol layering is a common technique for
simplifying networking designs. The work is divided into functional
layers, and separate protocols are assigned to perform each layer’s job.
This approach leads to a set of simple protocols, each taking care of a
few well-defined tasks.
A layer communicates with the layer above and below it, but it is not
aware of layers which are not directly adjacent to it. The key protocols
for the Internet are IP (Internet Protocol) and TCP (Transmission or
Transport Control Protocol), with IP operating on the layer below TCP.
The next reading describes how the Internet’s network model is
organized as four layers of protocols.
Reading 1.5
Connected: An Internet Encyclopedia; Programmed Instruction
Course, ‘DoD networking model’,
http://freesoft.org/CIE/Course/Section1/5.htm.
Note: A link at the bottom of this reading takes you to a discussion on
the topic of encapsulation. We refer to encapsulation a little later, so
please take a quick look at this link also.
In the previous reading, the topmost layer of the networking model is
where Internet services such as telnet, FTP, WWW and email (SMTP)
operate.
The lowest layer is responsible for passing data packets on to the
physical network cabling media. The popular protocols at the network
access layer are PPP (point-to-point protocols) for Internet connection
over regular phone lines and Ethernet protocol over Ethernet-based local
area networks.
IP is concerned with the routing of data between sender and recipient. It
does this by attaching a source and destination address to each packet,
like an envelope.
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TCP relies on IP to handle the details of getting data from one place to
another. On top of it, TCP provides mechanisms for establishing
connections between host computers, ensuring that data arrives in the
correct sequence, and retransmitting packets that are not received
correctly or promptly.
Depending on the literature, different names may be given to the layers in
the network model shown in Reading 1.5. To avoid confusion, I’d like to
present the model here again, along with other names that are commonly
used for the various layers.
Process or Application layer
Host-to-host or Transport layer
Internet layer
Network access or Physical
layer
Figure 1.13 Four-layer Internet network model
The way that the application, transport, Internet and physical layers work
together is called encapsulation. The application produces some data,
adds a header to it and hands off the result to the transport layer. The
transport layer adds another header, and hands the result off to the
Internet layer. It’s like putting a letter in an envelope, then putting that
envelope in a bigger envelope, and so on. On the receiving end, the
network software unpacks the envelopes one layer at a time until the
original data is handed to the receiving application.
Now that you have a high-level idea of how TCP and IP are related to
each other, let’s look at the details of how they work together.
Internet Protocol (IP)
IP manages the transfer of data across physically diverse networks. IP
transfers data in pieces, called packets or datagrams. Each packet is
encapsulated within an envelope of data describing where the packet
came from and where it wants to go. Packets are transferred from one
network to another according to the rules of the IP protocol, until it
arrives at its destination. Networks are fallible, though, so some packets
may be lost, delayed, or garbled along the way.
Unit 1
Back in Reading 1.4 ‘How Internet infrastructure works’, you learned
that routers are special gateway hosts which join different networks
together. Routers are like traffic cops who stand at intersections on the
Internet highway and decide if a packet is intended for a host within its
own network or needs to be routed to a different network. If the packet
needs to be forwarded to another network, the router uses its own routing
protocols to determine where to send it next.
IP specifies the formatting used to create packets and the addressing
scheme which gives every computer on the Internet a unique address. A
detailed discussion on the formatting specification of IP datagrams is
beyond the scope of this course, but the next figure is meant to illustrate
how an IP packet may look like once it is broken down into its
component fields.
VERSION
HEADER
LENGTH
SERVICE
TYPE
IDENTIFICATION
TIME TO LIVE
TOTAL LENGTH
FLAGS
PROTOCOL
FRAGMENT
OFFSET
HEADER CHECKSUM
SOURCE IP ADDRESS
DESTINATION IP ADDRESS
IP OPTIONS (IF ANY)
PADDING
DATA / PAYLOAD…..
Figure 1.14 Format of an IP datagram, containing the header and data area
Source: Comer 1991, 92.
Some of the key fields in the IP packet are:
•
Time-to-live (TTL) — limits the number of routers that a packet may
go through before reaching its destination. This prevents IP packets
from traveling on the Internet forever.
•
Protocol — lets the networking layer know what kind of transport
layer protocol is in the data segment of the IP packet. Common
transport layer protocols which use IP are TCP and User Datagram
Protocol (UDP).
•
Source and destination IP addresses — are the IP addresses of the
source and destination machines.
•
Data/Payload — contains the data which needs to be transmitted to
another computer. This data is passed down to IP by a transport
protocol such as TCP or UDP, as indicated by the protocol field.
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Activity 1.5
The Traceroute utility allows you to follow the path, or route, traveled by
a packet through the network. We will use this command to view the
number of hops that are traveled by a particular packet over the Internet.
1
If you are on a Windows machine, go to the the MS-DOS command
prompt and type the following command:
tracert www.google.com
If you are on UNIX, type the following command:
traceroute www.google.com
The output will display the intermediate routers that your packet has
to go through in order to arrive at its destination (www.google.com).
It also records the round-trip travel time for each router. My packet
had to travel more than 15 hops to reach Google.
2
Now let’s view the route to a Hong Kong-based server, namely,
OUHK. If you are on a Windows system, type this command:
tracert www.ouhk.edu.hk
If you are on UNIX, type this command:
traceroute www.ouhk.edu.hk
Here’s the output of traceroute against OUHK’s Web server.
C:\>tracert www.ouhk.edu.hk
Tracing route to sun17a.ouhk.edu.hk [202.40.157.186]
over a maximum of 30 hops:
1
2
3
4
5
6
7
8
30
10
20
20
20
20
20
20
ms
ms
ms
ms
ms
ms
ms
ms
20
20
20
20
20
20
30
20
ms
ms
ms
ms
ms
ms
ms
ms
20
20
20
20
40
30
20
30
ms
ms
ms
ms
ms
ms
ms
ms
203.99.136.128
shkdtswh01r1.so-net.com.hk [203.99.143.65]
203.99.143.161
agc2-RGE.hkix.net [202.40.161.189]
fe1-0-100M.ar2.HKG1.gblx.net [203.192.134.162]
ip-203.192.137.234.gblx.net [203.192.137.234]
sun25.ouhk.edu.hk [202.40.157.7]
sun17a.ouhk.edu.hk [202.40.157.186]
Trace complete.
Figure 1.15 Output of traceroute against OUHK’s Web server
My packets had to go through many fewer hops (only eight) in order to
reach OUHK as compared with Google. This explains why a website
should be hosted near its primary audience for better download times.
Unit 1
Traceroute is very useful for debugging problems within a network. If
you are unable to reach a destination server or if response time is slow,
you can use this utility to pinpoint problem areas and slow links.
IP addressing
We have learned that IP defines a system for assigning unique addresses
to all devices connected to the Internet. This is analogous to the system
used by Hong Kong’s postal service to locate residences and businesses
through street names and numbers. The next reading shows you how IP
addresses are formed.
Reading 1.6
Webopedia, ‘Understanding IP addressing’,
http://www.webopedia.com/DidYouKnow/Internet/2002/
IPaddressing.asp.
The reading describes how IP addresses are organized into two parts. The
first part identifies the network and the last part identifies a specific host
computer on that network. When data is routed on the Internet, the
network portion of the IP address is used to locate the correct network.
Once the data has arrived at the local network, the host portion of the IP
address is used to identify the correct computer within this network for
which the data is intended.
Let’s apply this concept to an example to see how IP addresses and
routing work together. Consider this IP address:
202.40.157.163
Starting from left to right in interpreting this address, we move from a
larger, more general area of the network (network 202) to a more specific
individual host on a smaller network (163).
Imagine this Internet address belongs to a host on the Open University of
Hong Kong’s local area network. A simplistic way of interpreting this
address is:
•
202 is a network that covers all of Asia;
•
40 is a network for the city of Hong Kong;
•
157 is the network containing all the computers for The Open
University of Hong Kong; and
•
163 is the individual host identifier of the computer on The Open
University of Hong Kong’s network.
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In terms of routing packets, the IP layer on the Asia network (202) only
needs to know how to send packets to the city of Hong Kong network
(40). It does not need to know anything about network 157 or host 163.
The city of Hong Kong network (40) only needs to know how to route
packets to the 157 network, and the 157 network only needs to know how
to route packets to host number 163. The way that the networks are
organized as a hierarchy limits the amount of knowledge that any one
routing node must have about the entire system of networks.
There is a special IP address that we will be using in future exercises:
127.0.0.1.
Network 127 is a specially designated network that is not owned by any
official organization. Individual computer hosts assume ownership of the
127 network address to manage their network resources. 127.0.0.1 is the
loopback address, a special address that computer hosts use to direct
TCP/IP traffic back to themselves. The loopback address is useful for
debugging and testing Internet services and we will use it in setting up
our own Internet services.
Activity 1.6
In this activity, you will view the current IP address assigned to your
machine. You must connect to the Internet first so that your computer
now becomes a host on the network. Depending on your Internet
connection, your ISP may assign you a static IP address or a dynamically
assigned IP address. A static address means your machine will always be
assigned the same IP address, while a dynamically assigned address will
be chosen from the pool of available addresses at the time.
1
If you are working on a Windows machine, you can view your
current IP address by typing IPCONFIG on the command line. If
this does not work on your system, try WINIPCFG.
2
If you are on a UNIX machine, type nslookup to display the IP
address of the machine.
Domain names
IP addresses such as 202.40.157.163 are difficult to memorize and
discuss, unless you’re a computer ☺. An alternate way of addressing
hosts using alphabetic or word-based names, called domain names, was
created. Examples of popular domains include amazon.com, yahoo.com,
and google.com. OUHK’s Web server corresponds to the domain name
ouhk.edu.hk.
Domain names are not just easier to remember, they also allow the
underlying IP addresses to be changed without affecting the name by
Unit 1
which the outside world knows them. You can think of the domain name
as the pseudonym or alias; the real name of the host computer is still its
IP address. It is up to the Domain Name Service to translate a given
domain name to its actual IP address.
The next reading describes how this translation process is accomplished.
Reading 1.7
Howstuffworks, ‘How Internet infrastructure works’ (by Jeff Tyson),
http://computer.howstuffworks.com/internet-infrastructure4.htm.
Note: Please only read the fifth section, ‘What’s in a name?’
Let’s conduct a few experiments to see how DNS maps domain names to
Internet addresses.
Activity 1.7
In this activity, we will use the nslookup utility to translate domain
names to their corresponding IP addresses. The same command can be
used on both Windows and UNIX systems.
1
In the command line, type nslookup www.yahoo.com. Here’s the
output on my system.
C:\>nslookup www.yahoo.com
Server: ns1.so-net.com.hk
Address: 203.99.142.8
Non-authoritative answer:
Name: www.yahoo.akadns.net
Addresses: 66.218.70.49, 66.218.71.86,
66.218.71.90, 66.218.71.95,
66.218.71.80, 66.218.71.92,
66.218.71.91, 66.218.70.48
Aliases: www.yahoo.com
Figure 1.16 Output of nslookup www.yahoo.com
It turns out that there are multiple IP addresses which correspond to
the domain name www.yahoo.com. This means that Yahoo uses more
than one Web server to handle the high volume of incoming traffic
on its website.
The nslookup command also returns the name and IP address of the
domain name server (DNS server) that was used to do the translation.
Can you tell what my DNS server is from the above display?
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2
Instead of using the domain name http://www.yahoo.com in the URL,
enter two of the numeric IP addresses above into your Web browser
to access Yahoo’s website (e.g., http://66.218.71.80 and
http://66.218.70.48). Both URLs should display Yahoo’s homepage,
which proves that they both map to the same domain name.
3
Now find out the IP address for www.kfbg.org.hk (the website of
Kadoorie Farm and Botanic Garden). In this case, the domain name
maps to a single numeric address.
Ports
Ports are numbers assigned to software applications or services running
on a computer. For example, a computer at an Internet Service Provider
(ISP) might be running a Web server, an email (SMTP) server and an
FTP server. How does a Web browser on a client PC say, ‘I want to
speak to the Web server,’ and an email client say, ‘I want to speak to the
email server.’? In order to identify the desired service, the client also
needs to know which port number on the remote server has been assigned
to the desired service.
Table 1.2 shows the list of established port numbers for common
services. For example, if a server machine is running a Web server and a
File Transfer Protocol (FTP) server, the Web server would typically be
available on port 80, and the FTP server would be available on port 21.
Clients connect to a service at a specific IP address and on a specific port
number. Once a client has connected to a service on a particular port, it
accesses the service using the protocol for that service.
Table 1.2
Protocol
Well-known port number assignments
Port
Description
echo
7
Allows one machine to ‘echo’ back the input
received from another machine
FTP
20, 21
Allows files to be exchanged between machines
telnet
23
Used to log in to remote machines
SMTP
25
Sends email between machines
HTTP
80
Used by Web browsers and servers
POP3
110
Transfers emails stored on a host machine to a
client machine
Some of these Internet services are described further in the next reading.
Unit 1
Reading 1.8
Web Developer’s Virtual Library, ‘Internet protocols’ (by Alan
Richmond), http://www.wdvl.com/Internet/Protocols/.
As you can see, the Web is only one of many services that run over the
Internet!
Transmission Control Protocol (TCP)
IP provides a best-effort service which strives to deliver packets to their
destination but does not guarantee that the delivery will be successful.
For an application such as the World Wide Web, IP’s best-effort level of
service is not enough.
Transmission (or Transport) Control Protocol (TCP) is needed in order to
manage and provide a reliable connection between two computers. You
can visualize a TCP connection between two hosts as a pipeline with two
endpoints. TCP segments or packets are put in one end and come out of
the other end. Data can be exchanged in both directions at the same time.
Socket
door
Application
writes data
Application
reads data
TCP
send buffer
TCP
receive buffer
Socket
door
segment
Figure 1.17 TCP is a connection-oriented protocol
Source: Kurose and Ross 2000.
TCP runs on top of IP and ensures that all IP packets which make up the
same message are transmitted safely, completely and correctly to their
destinations. TCP waits for the recipient to send back an
acknowledgement message for each packet that has been sent before it
sends out the next group of packets. If the recipient does not
acknowledge receipt within a designated amount of time, the client TCP
will resend the packet. The recipient may also send back an
acknowledgement message which asks the sender to retransmit the
packet if data has been corrupted or was not received in the correct
sequence.
The TCP protocol only runs in the source and destination host
computers. Intermediate network elements such as routers and bridges
merely forward IP packets without knowing which ones belong to the
same message or are part of the same ‘connection’. This demonstrates
protocol layering at work for you.
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Having taken a brief look at the TCP connection, let’s examine the TCP
segment structure. Similar to IP, the TCP segment consists of header
fields and a data field.
SOURCE PORT
DESTINATION PORT
SEQUENCE NUMBER
ACKNOWLEDGEMENT NUMBER
HLEN
RESERVED
CHECKSUM
OPTIONS (IF ANY)
CODE
BITS
WINDOW
URGENT POINTER
PADDING
DATA / PAYLOAD….
Figure 1.18 Format of a TCP segment with a TCP header followed by data
Source: Comer 1991, 183.
Some of the key fields are:
•
Source and destination port numbers — used by the two
communicating hosts.
•
Sequence number — shows the position of this data segment within a
group of segments. TCP breaks up a message into packets and then
labels each packet with a sequence number. This allows the two
communicating hosts to know what packets have been received and
which ones have not. It is also used to determine the order in which
packets should be reassembled.
•
Acknowledgement number — returned to the sender to acknowledge
receipt of a particular packet. It also informs the sender of the
sequence number of the next byte that the sender expects from the
recipient.
•
Checksum — used to verify that the data was not corrupted during
the transmission.
At this point you should understand TCP/IP, IP addresses, ports and
domain names. This should give you a basic understanding of how the
Web depends on the Internet’s protocols, infrastructure and services. Do
the following self-test to check your knowledge, and then check your
answers against those at the end of the unit.
Unit 1
Self-test 1.4
1
What is a protocol? Why are protocols important to the Internet?
2
Name three Internet services that use TCP/IP and describe what each
service does.
3
What is the role of IP within the TCP/IP suite of protocols?
4
What is the role of TCP within the TCP/IP suite of protocols?
5
What are some of the things that can go wrong when IP packets are
transmitted over a network?
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MT834 Web Server Technology
Web servers
We have seen that the Internet is built on open, public, standards such as
TCP/IP and that this openness permits a range of diverse networks to
easily join the Internet. The Web follows the Internet tradition and is also
built on open communication standards: the HyperText Transfer Protocol
(HTTP). Any program that implements the HTTP protocol can
participate in the World Wide Web. The standards and protocols are
general enough so that Web browsers and Web servers are implemented
on a wide variety of computers and written in a wide variety of
computing languages: C, C++, Java, etc.
This section presents a conceptual overview of how browsers and servers
communicate with each other, and how they perform their functions by
making use of lower-level services such as TCP/IP and DNS.
Role of browsers and servers
A browser is an HTTP client because it sends requests to an HTTP server
(Web server), which then sends responses back to the client. The
standard (and default) port for HTTP servers to listen for incoming
requests is port 80.
Server machine
running a
Web server
Your machine running
a Web browser
Your brower connects to
the server and requests
a page
The server sends back
the requested page
Figure 1.19 Web browser requesting documents from a Web server
Source: http://computer.howstuffworks.com.
Please note that there can be HTTP clients that are not Web browsers.
Programs written in a language such as Java or C can also issue HTTP
requests to a Web server, thereby acting as an HTTP client. For the
purposes of MT834, however, we will deal mostly with Web browsers
acting as Web clients.
Here are the basic steps that take place behind the scenes in order to
satisfy a Web request:
1
The browser gets the server name and the filename (including the
path) of the requested resource from the URL.
Unit 1
2
The browser asks a Domain Name Server to translate the server name
www.ouhk.edu.hk into an IP Address, which uniquely identifies the
Web server on the Internet.
3
The browser connects to the server on port 80.
4
The browser sends a request to the server which is written according
to the HTTP specification. The request will ask for the file
http://www.ouhk.edu.hk/index.html.
5
The server sends the HTML text for the webpage to the browser
using the HTTP protocol as well.
6
The browser interprets the HTML tags and displays the page on your
screen.
Steps 2 to 6 require the use of TCP/IP’s services. Let’s tie everything
together and see how these protocols are used in a typical Web surfing
session. The protocols are shown using the Internet four-layer model.
Host A
Host B
Application layer
(HTTP server)
Application layer
(HTTP server)
Transport layer (TCP)
Transport layer (TCP)
Internet layer (IP)
Internet layer (IP)
Physical layer
(Ethernet, FDDI,
LocalTalk, etc.)
Physical layer
(Ethernet, FDDI,
LocalTalk, etc.)
Cabling medium (twisted pair, fibre optic)
Figure 1.20 Web clients and servers use lower-level services in order to
communicate over the network
Installing a local Web server
Let’s now demonstrate how the HTTP communication protocol works on
your local computer by setting up a Web server and serving an HTML
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page to your Web browser. In the next activity you will set up the
Apache Web server, which is a cross-platform Web server (it compiles
and runs on a variety of Unix and Windows operating systems).
However, Apache was originally written to run on UNIX servers, and
new versions of Apache stabilize much faster on UNIX than Windows.
For the sake of reliability and security, it is recommended to run Apache
on the UNIX operating system.
The activities and practical work for the rest of this unit will assume that
you are using the Apache Web server on the Linux operating system.
Linux is an operating system that was developed under the GNU General
Public License and its source code is freely available to everyone. Linux
is often considered an excellent, low-cost alternative to other more
expensive operating systems.
Since there is no single company that controls Linux, several
organizations and individuals have developed their own ‘versions’ of the
Linux operating system, known as distributions. A Linux distribution is
based on Linus Torvalds’s Linux kernel, which contains the core
functions of the operating system.
We will assume that most of you are going to install Fedora Core 4 with
Intel-based CPUs. You can buy Fedora from a computer store or a book
shop, or you can download the software from many sites. One of them is
an ftp site:
ftp://ftp.cuhk.edu.hk/.1/Linux/distributions/fedora/core/4/i386/iso/
Note that we assume that you are using a 32-bit Intel processor. If not,
you need to go to other directories for your CPU. Then, you need to
download FC4-i386-discX.iso where X is 1 to 4. If you have a DVD
writer, you can just download the file FC4-i386-DVD.iso. Then use
these .iso files to burn four CDs or one DVD. Note that these .iso
files are CD images or DVD images, which means that you should use
‘burn image’ to burn them. If you are using Nero, the burn image option
can be found at
‘Recorder’ → ‘burn image’.
Before you install Fedora, you need to think about where to install it.
Linux can be installed on a dedicated system or on a dual-boot
configuration, where it co-exists with another operating system such as
Windows on the same machine. If you are planning to install Linux on a
dual-boot configuration, make sure you do a complete backup of your
existing system. There is always a possibility that you may lose all the
data contained on your drive when you work with the hard disk partition
table. Because of this, it’s usually recommended for beginners to install
Linux on a dedicated machine.
Therefore, the best solution is to install it on a new computer. The second
best is to install it on a new hard disk. If these options are not available to
you, the next best solution is to find a partition in your hard disk to install
it. Of course, the worst situation is that you cannot find anywhere in your
Unit 1
computer to install it. If you can afford it and your computer has space,
buy a hard disk to install Fedora.
The following reading is an installation guide for Fedora.
Reading 1.9
Fedora Core 4 Installation Guide,
http://fedora.redhat.com/docs/fedora-install-guide-en/fc4/.
The disks with FC4-i386-disc1.iso or FC4-i386-DVD.iso are
bootable. You should configure your PC to boot from the CD or DVD
drive. After booting the disk, you can start the installation process. If you
have enough disk space, I suggest you install all components of Fedora.
If you’re planning to connect your Linux box to the Internet, here are the
different ways to do so:
•
LAN;
•
dialup; and
•
broadband.
If you are using broadband, you need to know which technology is used
by your ISP.
Note that there are mainly two technologies used, namely PPPoE and
DHCP. If you require a password to get connected, you are most likely to
be dealing with PPPoE. Otherwise it will be DHCP. If you use a router to
connect to your ISP, then you just need to connect the Linux box to the
router. The following tells you how to configure the network connection.
For Fedora: Select RedHat (small icon on the left bottom corner) Æ
System settings Æ Network. You will be prompted for root’s password.
If you are using LAN, then you should double click on the network
connection to configure the IP address and the gateway. If you are using
dialup connection or PPPoE, you should click on the New button and
then select modem connection and xDSL connection respectively. Then
follow the instructions to complete the configuration. If you are using
DHCP or a router, you should double click on the Ethernet connection
and then configure the connection to use DHCP.
Apache should have been installed if you installed all components of
Fedora. Browse the pages on it through the loopback address, 127.0.0.1.
If you’ll recall from the section on IP Addressing, this is a specially
reserved address which directs TCP/IP traffic back to the local machine.
The loopback address corresponds to the name alias of localhost.
Entering http://127.0.0.1 or http://localhost in your browser will both
display the homepage of your local Web server.
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MT834 Web Server Technology
If your machine is currently on the Internet, your ISP may also have
dynamically assigned an IP address to your home computer. You cannot
depend on this address to be the same every time you connect. The
loopback address is the only fixed IP address that your Web server can
rely on to be available session after session, and this is what we’ll use to
test it.
HTML documents served by your localhost Web server can only be
retrieved by Web browsers running on the same machine. Web browsers
on the Internet will not be able to communicate with your localhost Web
server since they do not know your IP address.
If your computer is on a local area network and has a fixed IP address,
you will use that IP address to configure your Web server and then the
documents you placed on it will become available to the entire LAN (and
possibly, depending on your network set-up, the Internet).
If you are not familiar with Linux, the following Web site contains some
tutorials.
Reading 1.10
Lancom Technologies, ‘Hello Linux!’,
http://www.lancom-tech.com/hello-linux-crts.html.
Activity 1.8
1
Copy the homepage you created for ABC Books in Activity 1.1,
called abc_home.html into $(Apache_rootdir)\html where
$(Apache_routdir) is /var/www. You should also copy all
necessary images into a separate folder, such as
$(Apache_rootdir)\html\images. Ensure that the HTML
document refers to the images with the correct pathname.
2
Start the Apache server from the command line ‘/etc/rc.d/init.d/httpd
start’. You can stop or restart the server by replacing the word ‘start’
by ‘stop’ or ‘restart’ respectively.
3
Type in the URL http://127.0.0.1/abc_home.html into your Web
browser and see abc_home.html being served to you over the
loopback network to your Web browser.
4
Type in the URL http://localhost/abc_home.html into your Web
browser and retrieve the document. This demonstrates that 127.0.0.1
is the Internet address that maps to the localhost computer name.
Unit 1
From this demonstration you should understand how the Web client and
Web server communicate over the network when exchanging an HTML
document. Do the following self-test to check your understanding of the
Web’s system architecture.
Self-test 1.5
Describe the steps that take place so that your requested document can be
fetched and displayed on your computer.
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MT834 Web Server Technology
Summary
This unit has given you an overview of the components of the Web and
answered the question ‘What is the Web?’ You have seen that the Web is
the one of the most popular applications using the Internet as its
transmission medium. The three technologies that make up the Web are:
•
the Web’s document language: the HyperText Markup Language
(HTML);
•
the Web’s system for addressing and locating documents: Universal
Resource Locators (URLs); and
•
the communication language between the Web client and Web
server: the HyperText Transfer Protocol (HTTP).
In this first unit we explored HTML and URLs and you created your own
HTML document. HTTP will be discussed in more detail in Unit 2 Web
servers and HTTP.
We also discussed the Internet — its characteristics, design features, and
how it is related to the Web. The Internet is the underlying network that
carries Web traffic consisting of HTTP request and response messages.
This network uses a unique design meant to fulfill the requirements of a
wartime network. We also examined the TCP/IP network protocol used
to transmit data over the Internet. Essentially, HTTP messages are broken
down into packets, routed individually over the network, and then
reassembled at their destination.
To appreciate how the Internet, Web browsers and Web servers interact,
you installed and configured an Apache Web server to serve your HTML
document. The next unit in this course examines Web server software in
detail and looks at the step-by-step mechanism by which a Web browser
‘talks’ to a Web server according to the HyperText Transfer Protocol
(HTTP).
Unit 1
Suggested answers to self-tests
Self-test 1.1
1
In the client-server model, an end-user relies on a program (the
client) to communicate with an application residing on a remote
machine (the server), in order to retrieve the requested resource. The
Web follows this model, splitting the system’s functions between two
tiers — the client and the server.
From the client’s point of view, all that’s needed is a Web browser
and an Internet connection in order to get on the Web. From the
server’s point of view, what’s needed is a machine that’s connected
to the Internet, runs Web server software and hosts the required
documents.
2
The Web uses the Internet as its underlying network medium. Data
exchanged between Web clients and servers travel over the Internet.
However, the Internet and the Web are not one and the same. There
are many other services aside from the Web which run over the
Internet.
Self-test 1.2
1
Three new technologies were created to build the Web:
•
The HyperText Markup Language (HTML) defines how
webpages and hypertext links are written.
•
Universal Resource Locators (URLs) define the Web’s system
for addressing and locating documents. Hypertext links contain
URLs.
•
The HyperText Transfer Protocol (HTTP) is the communication
language or protocol between the Web client (the Web browser)
and the Web server. It describes how clients make requests for
information and how servers respond to them.
2
Hypertext is a system of a collection of documents that are associated
through active links. When a user chooses a link, that link is
followed, and the document that link pointed to is fetched and
displayed. Hypertext is a non-linear text system that creates an
‘information space’.
3
A URL is the address of a unique location of an Internet resource.
For example, http://www.lycos.com is the URL for the Lycos search
engine and ftp://ftp.ncsa.uiuc.edu is the URL for NCSA’s FTP site.
4
Hypertext links are represented as anchor tags in HTML. An anchor
tag takes this form in HTML:
<A HREF="http://www.ouhk.edu.hk"> The Open
University of Hong Kong</A>
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MT834 Web Server Technology
5
An inline image takes this form in an HTML document:
<IMG SRC = "picture.gif">
6
These HTML tags should always be in an HTML document:
<HTML></HTML>
<HEAD><TITLE></TITLE></HEAD>
<BODY></BODY>
Self-test 1.3
1
There are two design characteristics that can help the Internet
withstand catastrophic attacks in wartime. First, it is decentralized.
Computers can communicate directly with each other without having
to go through a central node. This ensures that communication can
still take place even if some machines are destroyed. Second, the
Internet is a packet-switching network which offers multiple,
redundant routes between two endpoints. This ensures that packets
can still be routed between two hosts even if certain sections of the
network are rendered inoperable.
2
The Internet is a global network that is made up of many smaller
networks. These smaller networks connect with each other to form
bigger and higher-level networks. There is no overall controlling
network. Instead, these higher-level networks connect to each other
through Network Access Points (NAPs). The Internet is a collection
of huge networks which implement the same protocols and agree to
route data traffic to each other at these Network Access Points.
Self-test 1.4
1
A protocol is a set of conventions or rules specifying how each party
should communicate. The details of Internet protocols are public
domain, open standards. A network joins the Internet if it follows the
communication rules specified in the TCP/IP protocol. An individual
host computer can join the Web if it follows the communication rules
specified in the HTTP protocol.
2
These are a few Internet services that use TCP/IP:
•
Domain Name Service (DNS) is an Internet service which maps
Internet names to Internet addresses. Given an Internet domain
name DNS will return an Internet address. Given an Internet
address DNS will return an Internet domain name.
•
File Transfer Protocol (FTP) is an Internet service which allows
users to copy files from one computer to another across the
Internet.
Unit 1
•
Telnet is an Internet service which enables a person to set up a
connection, log in, and conduct an interactive session with a
remote computer on the Internet.
•
The World Wide Web is a hypertext-based Internet service. The
WWW uses URLs as its addressing system and HTTP as its
communication protocol.
3
Internet Protocol (IP) handles the addressing and coordinates the
routing of packets across multiple Internet nodes.
4
TCP establishes a connection from one point on the Internet to
another point on the Internet. Once the connection is established TCP
is responsible for breaking the data up into packets and ensuring the
reliable transfer of the packets over the network. TCP is responsible
for detecting and correcting errors in the data transfer process. IP
routes the packets across the nodes of the Internet. IP is the packet
mover of the Internet.
5
Packets can be lost or destroyed when network hardware fails, or
when networks become too congested with traffic. Even when
packets make it to their destination, they may be delivered out of
order, after a long delay, or with duplicate copies. This is why a
reliable delivery service such as TCP is needed on top of an
unreliable, connectionless packet delivery service such as IP.
Self-test 1.5
1
The browser extracts the server name and the file name (including
the path) from the URL.
2
The browser asks a Domain Name Server to translate the server name
into a corresponding IP address. If the URL includes the numeric IP
address instead of the server name, this step is not needed.
3
The browser establishes a connection to the server at its IP address
on port 80.
4
The browser sends an HTTP request to the server. The request
includes the path name and file name of the requested resource.
5
The server sends the HTML text for the webpage to the browser
within an HTTP response message.
6
The browser receives the HTTP response over the network. It
interprets the HTML tags and displays the page on the screen.
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References
Andrews, J, Cutura, T, Hudson, K and Spivey, L A (2000) I-Net+ Guide
to Internet Technologies, Boston: Course Technology.
Comer, D E (1991) Internetworking with TCP/IP: Volume I; Principles,
Protocols, and Architecture, Upper Saddle River, NJ: Prentice Hall.
Connected: An Internet Encyclopedia; Programmed Instruction Course,
http://freesoft.org/CIE/Course/index.htm.
Kurose, J F and Ross, K W (2000) ‘3.5 connection-oriented transport:
TCP’, http://www-net.cs.umass.edu/kurose/transport/segment.html.
Wainwright, P (2002) Professional Apache 2.0, Wrox Press Ltd.
Web Developer’s Virtual Library, http://www.wdvl.com.
Yeager, N and McGrath, R E (1996) Web Server Technology: The
Advanced Guide for World Wide Web Information Providers, San
Francisco: Morgan Kaufmann.